This invention relates to underwater flux-cored arc welding and, more particularly, to flux formulations for such welding for performing repairs to nickel-based and austenitic stainless steel materials.
Most internal components of a reactor pressure vessel are made of nickel-based and austenitic stainless steel materials. Because of the high radiation field and inaccessibility of components in the lower section of such a vessel, automated underwater wet welding is considered the most practical method of performing repairs to materials for such internals. The mechanical methods presently being used for this type of repair application produce crevices, and this promotes concerns about so-called intergranular stress corrosion cracking (IGSCC), crevice corrosion and pitting. A welded repair would offer a permanent fix without creating additional crevice locations. So-called underwater wet-shielded metal arc welding (SMAW) has been successfully used to repair components in radiation areas in upper sections of reactor pressure vessels, although this is a manually operated process not feasible for remote applications.
The flux-cored arc welding process (FCAW) has been developed because of its ease of automation, out-of-position welding proficiency and self-shielding capabilities. The flux-cored arc welding process is characterized by the use of a continuously fed consumable welding wire comprised of a metal sheath filled with flux and alloying elements. A flux formulation for a self-shielding flux-cored wire is used to form a slag and to produce a shielding gas to protect the molten transfer and weld puddle, similar to the SMAW process. Conventional filler materials, however, include harmful halogen-containing components, such as calcium fluoride (CaF.sub.2) and cryolite (Na.sub.3 AlF.sub.6) A typical prior art self-shielded flux chemistry contains:
50-70%: Rutile, Titania (TiO.sub.2), PA1 5-20%: Zirconium oxide (ZrO.sub.2), PA1 2-7%: Silicon oxide (SiO.sub.2), PA1 1-3%: Cryolite (Na.sub.3 AlF.sub.6), PA1 10-30%: Potassium titanate (K.sub.2 O/TiO.sub.3 at ratio of 3:1).
The halogens that dissociate in the welding arc form free halogen ions, and the halogen ions introduced into water attack passive corrosion-resistant layers typical of austenitic stainless steel and nickel-based alloys, promoting corrosion cracking.
It is therefore an object of the present invention to provide a self-shielded flux formulation to accommodate underwater wet flux-cored arc welding process for repairing components of a reactor pressure vessel.
It is another object of the invention to establish a flux formulation with high underwater weldability without including halogen-containing elements.